I digivolve into this card while I have 4 or more security cards. While this Digimon's [When Digivolving] effect is pending activation, one of my other effects reduces my security cards to 3 or fewer. Do I?
Yes, you do. This effect behaves the same way as an effect that reads "If you have 3 or fewer security cards", so the condition is checked when the effect would be activated.